Here's an infographic that shows off the 25 most expensive bad sci-fi movies ever made. The only movie on the list that I don't think belongs on there is Speed Racer, because I don't care what anyone says, that movie was freakin' awesome.
According the Geek Twins,
These are rated by the following criteria: Budget (or cost to make the movie) and the current score from the rating aggregate site Rotten Tomatoes.
Some movies didn't get in because they didn't fit the criteria. For example, the Ed Wood film Plan 9 from Outer Space is arguably horrible, but only cost $60,000. Spider-Man 3 was expensive at $240,000,000, but got a 67% meaning it wasn't considered that bad.